As signal amplification methods without using any enzyme, there have been reported a signal amplification method using a pair of oligonucleotides (hereinafter, referred to as HCPs) represented by the following chemical formulae (1) and (2) to form a self-assembly substance (polymer) of the HCPs (hereinafter, referred to as a PALSAR method) and a method of detecting genes using the method (Patent Documents 1 and 2, etc.).

In the formulae (1) and (2), the region X and X′, the region Y and Y′, and the region Z and Z′ are complementary nucleic acid regions capable of hybridizing with each other, and a self-assembly substance represented by the following chemical formula (3) is formed by binding plural pairs of HCPs. In the present specification, the signal probe polymer refers to the self-assembly substance formed from HCPs. Meanwhile, the assist probe refers to a probe, which has both a sequence complementary to that of a target gene to be detected and a sequence complementary to that of an HCP, and plays a role in linking the target gene to a signal probe polymer.

Meanwhile, Patent Document 3 discloses a method of detecting a target gene on a DNA tip by using the PALSAR method. Patent Documents 1 to 3 disclose methods of sensitively detecting a target gene by forming a complex of a target gene and a self-assembly substance to detect the self-assembly substance. A method of forming a signal probe polymer on a target gene includes a method of designing an HCP so as to have a sequence complementary to that of a target gene, and a method of using an assist probe. Of those, the method of forming an assist probe has an advantage of being capable of detecting a plurality of genes with a pair of HCPs by preparing a plurality of assist probes modified so as to have different sequences complementary to that of a target gene.
In each of Patent Documents 1 to 3, an assist probe having a sequence complementary to that of one region in an HCP is illustrated, but an assist probe capable of sensitively detecting a target gene have not been clarified.
[Patent Document 1] JP 3267576 B
[Patent Document 2] JP 3310662 B
[Patent Document 3] WO 2003-029441
[Patent Document 4] WO 2004-074480
[Patent Document 5] WO 2004-072302